The present invention relates in general to a light detecting and amplifying circuit and in particular to a light detecting circuit that provides solar cancellation.
Photocells are devices that produce an output current generally proportional to the irradiance of the light impinging thereon. As such, photocells provide a convenient means to measure the intensity of electromagnetic radiation in the visible and infrared regions. However, the output signal generated by the photocell is typically weak; thus some sort of amplification and/or signal conditioning is used before further processing of the output current by additional signal processing circuitry.
Photocells generally comprise an intrinsic depletion layer sandwiched between an n-type doped region (the cathode) and a p-type doped region (the anode). If the photocell is reverse biased (a positive voltage applied from the cathode to the anode), the depletion area increases and the capacitive effects of the photocell decrease. A typical reverse bias light receiving circuit 10 is illustrated in FIG. 1. Light impinging on the photocell 12 causes a current (IPC) 14 to flow from the anode of the photocell 12 into the load resistor (RL) 16. The current 14 is typically weak and varies depending upon the intensity of light on the photocell. The load resistor 16 is provided to convert the current 14 into a voltage (VPC) 20. Further, amplifier 22 is provided to amplify the voltage across the load resistor 16.
Photocells typically exhibit a parasitic capacitance that can be expressed by the well-known formula:   C  =                    ϵ        s            ⁢      A              x      T      
Where xcex5s is the permitivity of substrate, A is the junction area, and XT is the width of the depletion region.
It is desirable to minimize this capacitance to improve the speed and response time of the photocell. It should be clear from the above formula that increasing the width of the depletion region of the photocell reduces the parasitic capacitance. This is accomplished by applying a reverse voltage between the cathode and anode. As such, a bias voltage (VBias) 18 is applied to the cathode of the photocell.
As a practical matter, the selection of a value for the load resistor 16 can prove a limiting factor in the circuit performance. For example, the load resistor 16 forms a parallel circuit with the input impedance of the amplifier 22. An appreciable amount of the output of the photocell 12 can thus be lost across the load resistor 16 due to loading effects if the resistance value of the load resistor 16 is chosen too small. The amount of signal loss due to the loading effects of the load resistor 16 is the percentage of the load resistor 16 in parallel with the transimpedance gain of the amplifier 22. On the other hand, higher resistance values result in higher resistor thermal noise, which can compromise the accuracy of the output by the photocell 12, especially when the output current is weak. Also, the higher the value of the load resistor 16, the more limited the dynamic range of the photocell 12. This is seen by the observation that should the voltage 20 across the load resistor 16 (computed from Ohm""s law as the photocell output current 14 times the resistance value of the load resistor 16), exceed the bias voltage 18, the photocell 12 will no longer be reverse biased, and the capacitance of the photocell increases. To prevent this from happening, the bias voltage 18 is known to exceed 50 volts in some applications. Such a solution is inefficient, especially when designing the circuit for battery powered portable devices.
Another disadvantage of the prior art circuit of FIG. 1 lies in the observation that the reverse voltage between the cathode and anode of the photocell, and thus the parasitic capacitance of the photocell, changes as the intensity of light impinging on the photocell changes. The reverse voltage that biases the photocell is the bias voltage 18 minus voltage 20 across the load resistor 16. As pointed out above, as light intensity impinging on the photocell 12 increases, the photocell output current 14 increases, and thus the voltage 20 increases. This lowers the effective reverse voltage, and thus increases the capacitance of the photocell. As a result, response time of the photocell can become sluggish and the photocell may become ineffective at capturing short duration pulses of light.
Yet another disadvantage of the circuit of FIG. 1 is that any noise in the power supply that provides the bias voltage 18 is seen as an output of the photocell 12 and amplified by the amplifier 22.
Accordingly, there is a need for a circuit that provides a constant reverse voltage to a photocell. Further, there is a need for a circuit that can maintain the constant reverse voltage by using a low voltage supply.
The present invention overcomes the disadvantages of previously known receiver circuits by providing a circuit that maintains a constant reverse voltage across one or more photocells. This maintains the capacitance of the photocell at a substantially constant value, and fast photocell response times are realized. Further, the present circuits reduce the amount of signal loss due to loading at the input stage of the amplifier, and effectively remove noise, including ambient conditions such as daylight.
In accordance with one embodiment of the present invention, a single operational amplifier is used to maintain a photocell at a constant reverse voltage. The photocell is connected to the circuit such that the anode is coupled to ground, and the cathode is tied to the inverting input of an operational amplifier (op-amp). The non-inverting input of the op-amp is tied to a reference voltage, which is adjusted to the desired bias voltage. Further, an inductor is provided in a negative feedback loop between the inverting input and output of the op-amp. The photocell is also coupled to a transimpedance amplifier through a capacitor. The inductor is seen by the photocell as a low impedance load for low frequency signals. Thus, the effects of daylight, which are observed by the photocell as d.c. or low frequency signals are, effectively buffered. Further, the photocell sees the inductor as a high impedance load at high frequencies. Thus a substantial portion of the signal of interest from the photocell is delivered to the transimpedance amplifier.
According to another embodiment of the present invention, a photocell is connected to the circuit such that the anode is coupled to ground, and the cathode is tied to the inverting input of an operational amplifier (op-amp). The non-inverting input of the op-amp is tied to a reference voltage, which is adjusted to the desired bias voltage. Further, a parallel combination of an inductor and a capacitor is provided in a negative feedback loop between the inverting input and output of the op-amp. The photocell is also coupled to a transimpedance amplifier through a capacitor.
The parallel combination of the inductor and capacitor are tuned such that the feedback loop of the op-amp provides a high impedance load at frequencies of interest. Thus a substantial portion of the signal from the photocell is delivered to the transimpedance amplifier. Further, the parallel combination of the inductor and capacitor provide a low impedance load at frequencies of no interest. The inductor is seen by the photocell as a low impedance load for low frequency signals. Thus the effects of daylight, which are observed by the photocell as d.c., or low frequency signals are effectively buffered by the inductor. The capacitor placed in parallel with the inductor is seen by the photocell as a low impedance load for very high frequency signals. Thus high frequency noise is effectively buffered and reduced from appearing at the output of the transimpedance amplifier.
It is thus an object of the present invention to provide a circuit that supplies a photocell constant reverse voltage that remains substantially constant, irrespective of the photocell output current.
It is an object of the present invention to provide a circuit that actively filters the effects of daylight from the receiver.
It is an object of the present invention to provide a circuit that buffers the output current from the photocells when the output comprises frequencies of no interest.
It is an object of the present invention to provide a circuit having a very high impedance buffer at the frequencies of interest such that there is minimal signal loss in the transfer of the signal of interest from the photocell to the amplifier.